Lect 5: Fluids & Solids Flashcards
matter that is either liquid or gas; molecules bond weakly, break, and reform because of higher Kinetic Energy; create permanent Forces outward (Normal to the surface); Permanent force withstands the Force parallel to the surface; Matches shape of container; Gravitational Force causes flat top in liquids
Fluid
property of fluids that can be externally viewed and measured; Quantity can change
Ex: mass and Energy
extensive properties of fluids
properties of fluids that are intrinsic; Independent of quantity;
Ex: density, pressure
Intensive
the “heaviness of a fluid”; the mass in a specific volume; changing the amount of the fluid does NOT change this; this is changed only by a change in volume without a change in mass; Solid measurement SI unit = kg / m^3;
density (rho)
Equation: Density
Density (rho) = m / v
the density of a subject compared to the density of water; ratio quantity
specific gravity
If the density (specific gravity) of the substance is less than 1, the substance is _____ than water
lighter
if the density (specific gravity) of a substance is greater than 1, the substance is _______ than water
heavier
Equation: Specific gravity
Specific Gravity = density (substance) / density (water)
Specific gravity / density of water
1000 kg / cm^3
1 g / cm^3
a Force per unit area; SI unit = Pascals (Pa)
Note: also “stored” energy per unit area
pressure
results from impulse (change in momentum) or F(collision) * time of molecular collisions; change in momentum is the average number of collisions / time of collisions and the surface area of the object in the collisions; SI unit = Pascals (Pa)
fluid pressure
Equation: Pressure
P = F / A
a fluid at rest only experiences a force ______ to the surface
perpendicular
pressure of a disc submerged in water
P = weight (m*g) of fluid ABOVE the disc / area of the disc
Pressure is ______ of the area chosen
independent
Equation: Pressure of a Fluid at rest in a sealed container w/ uniform density
P = (density, rho) * g * y
Note:
y = depth of the fluid
g = gravitational constant
Equation: Pressure of a fluid at rest in an open container
P = density (rho) * g * y + P(atm)
Pressure of the atmosphere
101,000 Pascals
1 atm
Pressure of a fluid ______ as y (depth of fluid) goes down. Why?
decreases; bc there are fewer molecules above the object causing less weight
The pressure compared to local atmospheric pressure;
Gauge pressure
“Negative” pressure
always refers to gauge pressure; Pressure for the system is less than that in the atmosphere
pressure measured relative to a vacuum as 0
absolute pressure
Equation: Absolute pressure
P(abs) = P(g) + P(atm)
NOTE: P(g) is Gauge pressure
Each point in an enclosed fluid must bear any increase in pressure
pressure applied anywhere to an enclosed incompressible fluid will be distributed undiminished throughout that fluid
Pascal’s Principle
a simple machine that works via Pascal’s Principle; Force on Piston 1 applies pressure on incompressible fluid; All pressure is transferred to Piston 2 (w/ a larger Area)–Force is proportionately greater but acts over a larger distance
So, ∆P1 = ∆P2
hydraulic lift
an ideal machine does not change ______
Work
the upward force acting on a submerged object;
Displaces volume
due to a difference in pressure
equal to the weight (m*g) of the fluid displaced
Buoyant force: F(b)
Equation: Buoyant force
density (rho) of the fluid * V(g)
Note: V(g) = volume displaced
Equation: Floating Object
submerged part of the object = density of the object / density of the fluid
Note: If floating in water, the ratio = specific gravity of that object
motion in a fluid at rest
contributes to the fluid pressure
random translational motion
motion that is shared equally by all molecules at a point in the fluid; motion of the fluid as a whole; does not contribute to fluid pressure
uniform translational motion
Equation: Velocity of fluid coming from a spigot
V = √2gh
a fluid with no viscosity that is incompressible, no turbulence, irrotational flow
ideal fluid
measure of a fluid’s resistance to Force that is not perpendicular to the surface; fluid’s tendency to resist flow
Ex: syrup > water
viscosity
fluid w/ uniform density
*Assume this for MCAT
incompressible fluid
steady flow of a fluid; all fluid flowes through a fixed point will have the same VELOCITY
Laminar flow
Equation: Continuity equation (Volume flow rate)
Q = A * v
Note: Q = volume flow rate
rate at which a volume of liquid moves through a pipe
volume flow rate (Q)
equals a fluid’s volume flow rate * density
mass flow rate (I)
Equation: Mass flow rate (I)
I = density (rho) * Q = density (rho) * A * v
For an Ideal fluid, the ______ is constant, and Area and Volume are ______ related
Flow rate is constant inversely related (narrow pipe = high velocity)
Equation: Bernoulli’s Equation
P + density (rho) * g * h + 1/2 * density (rho) * v^2 = K
Note: K = constant specific to a fluid in a given situation of flow; h = distance ABOVE some arbitrary point
given one continuous idea flow;
sum of its three terms is a CONSTANT at any point in the fluid
similar to conservation of energy: dividing any term by Volume gives units of energy
Bernoulli’s equation
Equation: gravitational potential energy per unit volume of a fluid
m * g * h / V
Note: Second term in Bernoulli’s equation
Equation: kinetic energy from the uniform translational motion of the molecules in a fluid per unit volume
(1/2 * m * v^2) / V
Note: third term in Bernoulli’s equation
Equation: Energy per volume from the random motion of the molecules in a fluid
P / V
Note: First term from Bernoulli’s equation
Uniform translational energy borrows energy from _____________. Pressure goes down
Random translational kinetic energy
Pressure and velocity are ________ related in ideal fluids
inversely; As velocity goes down, pressure goes up
predicts deviation from ideal fluids; Ex: drag
Non-ideal (Real) fluids
occurs at the fluid - object interface most often; force is working against the flow
drag
the intensity of the intermolecular forces per unit length in a fluid
temperature dependent
depends on which type of fluid is interfacing
surface tension
the force on a fluid causes water droplets by maximizing the surface area
intermolecular forces
allows a fluid to be pulled up a thin tube
capillary action
intermolecular force that causes surface tension
cohesive force
force between molecules of the tube and the fluid molecules
adhesive force
type of matter in which atoms and molecules are held together rigidly; molecules change dimensions by stretching / compressing but not breaking
solids
force applied to a solid object / Area over which the Force is acting
i.e. What is done to an object
stress
Equation: Stress
Stress = F / A
SI Unit = N / m^2
the fractional change in an object’s shape
i.e. how an object responds
strain
Equation: Strain
strain = ∆dimension / original dimension
the maximum stress point an object can take; beyond this point, the object loses its original shape
yield point
when stress exceeds the yield point for an object
Fracture point
3 Stress modules for solids
young’s module (E) - tensile strength
shear’s module (G) - shear stress
Bulk module (B) - compression / expansion
Equation: Young’s module
E = (F/A) / (∆h /ho)
Equation: Shear’s module
G = (F/A) / (∆x / ho)
Equation: Bulk module
B = ∆P / (∆v / vo)
Solids ______ when heated due to more molecular vibrations. May be either linear or volume
Expand
Equation: Modulus of elasticity
Modulus of Elasticity = Stress / Strain
